1. Field of the Invention
This invention relates to vibrators for injecting seismic signals into the earth and particularly to an apparatus for preventing stress fatigue in vibrator stilt structures.
2. Description of the Prior Art
Conventional seismic vibrators consist of a well-known ground-contacting baseplate, a dual-ended actuator assembly with one end connected to the baseplate, and a support frame connected to the baseplate and to the other end of the actuator assembly. The baseplate is attached by well known hydraulic columns to a support vehicle such as a truck.
The support frame, also known as a stilt structure, is usually constructed of I-beam or channel steel in the form of a rectangular arch. The frame provides upright support for the actuator and also reinforces the coupling of the actuator to the baseplate.
Conventional vibrators usually operate within the range of 5-80 Hz. The large mass of the baseplate/stilt structure system does not limit and distort the amplitude of relatively low frequency signals. However, when operated at frequencies in the range of 80-150 Hz and above, conventional vibrators tend to inject attenuated and distorted signals. Distortion of the vibrator output signal at the higher frequencies results from mechanical resonances that develop in the baseplate and stilt structure. The baseplate may resonate in such a manner that the displacement at the center of the plate may be substantially out of phase with respect to the displacement of the edges thereof.
In order for high-frequency vibrators to operate efficiently, it is necessary to reduce the weight of the baseplate and stilt structure so as to improve the frequency response of the spring-mass system (baseplate and stilt structure). It is also essential to provide structural stiffness in the baseplate and stilt structure to minimize mechanical resonance development.
U.S. Pat. No. 4,406,345 issued to Fair teaches a construction design for high-frequency vibrators. The design is intended to provide structural stiffness while also reducing the weight of the baseplate assembly.
Fair replaces the conventional I-beam-type stilt structure with a frusto-conical vibrator housing that completely encloses the actuator assembly. The shape of the steel housing provides maximum support with less weight. The bottom flange of the housing is firmly bolted to the baseplate adding structural stiffness to the plate and housing. FIG. 1 of Fair's patent illustrates a typical construction of a high-frequency vibrator.
To further increase the efficiency of high-frequency vibrators, the baseplate and housing may be fabricated of light-weight high-strength materials such as aluminum or titanium alloys or plastic laminates. However when using dissimilar materials in major structural elements of vibrator construction, in contrast to Fair's all-steel construction, a high rate of structural failures occur.
A majority of structural failures occur as a result of variable initial prestress within the structure. This is particularly true when the dissimilar materials are characterized by different coefficients of thermal expansion. For example, aluminum has an expansion coefficient that is 2 times that of steel. When aluminum and steel structures are rigidly bolted together, as they are in a high-frequency vibrator, a moderate temperature change creates a large stress due to differential expansion or contraction of the two metals which results in a strain or deformation of the respective structures. Eventually, structural failure occurs as a result of fatigue.
It is an object of this invention to prevent vibrator structural failures associated with thermally-induced strains.